1. Field of the Invention
This invention relates to semiconductor devices and more particularly to a radiant energy detector incorporating a heterojunction.
2. Description of the Prior Art
Passivation of mercury cadmium telluride surfaces and exposed junctions, particularly in materials responsive to radiant energy in the eight to twelve micrometer wavelength is a difficult problem. The technology of thick native oxide growth on mercury cadmium telluride is not available.
In U.S. Pat. No. 3,799,803, which issued in March, 1974, to H. Kraus and B. H. Breazeale, the passivation of a semiconductor surface namely mercury cadmium telluride is described by means of a hydrogen peroxide rinse to remove contaminants from the surface.
In U.S. Pat. No. 3,845,494, which issued in October 1974 to J. F. Ameurlaine and G. D. Cohen-Solal, a mercury cadmium telluride photovoltaic detector is described covered by a continuous film of material such as a metallic sulfide or selenide for preventing outward diffusion of mercurcy vapor from the detector. The film or coating which is impervious to mercury may include zinc sulfide, zinc selenide or arsenic pentaselenide.
In U.S. Pat. No. 4,132,999, which issued in January 1979 to J. H. P. Maille, a pn junction is formed in mercury cadmium telluride material by diffusing mercury through a protective layer of cadmium telluride. A mask of zinc sulfide defines the boundaries of the pn junction formed beneath the layer of cadmium telluride.
In U.S. Pat. No. 4,137,544, which issued in January 1979 to T. Koehler, a mercury cadmium telluride diode is formed by an ion implating acceptor impurities such as phosphorus, antimony or arsenic into an n-type substrate of mercury cadmium telluride. In addition, a cumulation layer is formed at the surface of the mercury cadmium telluride substrate to surround the p-type region formed by ion implantation. The ion implantation is performed right through a passivation layer such as anodic oxide. An additional mask is provided above the passivation layer to define the p-region. The resulting pn junction is formed within the mercury cadmium telluride substrate and extends to its surface which is protected by the passivation layer. The diode is particularly useful for detection of radiation in the range from 8 to 14 micrometers.
In U.S. Pat. No. 4,170,666, which issued in October 1979, to R. K. Pancholy, G. J. Kuhlmann and D. H. Phillips, the effective surface recombination velocity of III-V compound semiconductors is reduced by providing a native dielectric passivation layer on the semiconductor and by inducing a potential in the vicinity of the semiconductor-dielectric interface which repels approaching minority carriers. A layer of gallium arsenide phosphide is formed by converting the surface of gallium arsenide. Gallium arsenide phosphide has a higher energy band gap than gallium arsenide and increases the energy which minority carriers must possess in order to reach the interface of the gallium arsenide phosphide and a gallium phosphate oxide passivation dielectric as shown in FIG. 3.
It is therefore desirable to fabricate pn junctions in mercury cadmium telluride substrates with reduced surface leakage current and higher R.sub.o A products by forming the pn junction underneath a protective surface.
It is further desirable to fabricate a heterojunction photodiode which is responsive to radiant energy in the range from eight to twelve micrometers.
It is further desirable to passivate material such as mercury cadmium telluride which has a composition for an energy band gap sensitive to radiation in the range from eight to twelve micrometers by covering the exposed material with a layer of material such as mercury cadmium telluride having a composition for a greater energy band gap such as being sensitive to radiant energy in the three to five micrometer wavelength range.
It is further desirable to provide a heterojunction diode having a mesa structure wherein the edges of the pn junction of the narrowest band gap material are buried or covered by semiconductor material of the wider band gap.
It is desirable to provide a heterojunction diode having a planar structure wherein the pn heterojunction of the narrowest band gap material is buried or covered by the semiconductor material of the wider band gap.